Selasa, 10 Maret 2009

Again, please bear with me... Let's start with simple pin-out diagrams of the twotypes of UTP Ethernet cables and watch how committees can make a can of wormsout of them. Here are the diagrams:

Note that the TX (transmitter) pins are connected to corresponding RX (receiver)pins, plus to plus and minus to minus. And that you must use a crossover cable toconnect units with identical interfaces. If you use a straight-through cable, one ofthe two units must, in effect, perform the cross-over function.Two wire color-code standards apply: EIA/TIA 568A and EIA/TIA 568B. The codesare commonly depicted with RJ-45 jacks as follows (the view is from the front of thejacks):

If we apply the 568A color code and show all eight wires, our pin-out looks like this:

Note that pins 4, 5, 7, and 8 and the blue and brown pairs are not used in eitherstandard. Quite contrary to what you may read elsewhere, these pins and wires arenot used or required to implement 100BASE-TX duplexing--they are just plainwasted.However, the actual cables are not physically that simple. In the diagrams, theorange pair of wires are not adjacent. The blue pair is upside-down. The right endsmatch RJ-45 jacks and the left ends do not. If, for example, we invert the left sideof the 568A "straight"-thru cable to match a 568A jack--put one 180° twist in theentire cable from end-to-end--and twist together and rearrange the appropriatepairs, we get the following can-of-worms:

This further emphasizes, I hope, the importance of the word "twist" in makingnetwork cables which will work. You cannot use an flat-untwisted telephone cablefor a network cable. Furthermore, you must use a pair of twisted wires to connect aset of transmitter pins to their corresponding receiver pins. You cannot use a wirefrom one pair and another wire from a different pair.Keeping the above principles in mind, we can simplify the diagram for a 568Astraight-thru cable by untwisting the wires, except the 180° twist in the entirecable, and bending the ends upward. Likewise, if we exchange the green and orangepairs in the 568A diagram we will get a simplified diagram for a 568B straight-thrucable. If we cross the green and orange pairs in the 568A diagram we will arrive at asimplified diagram for a crossover cable. All three are shown below.